Conditions of the formation of thermomineral waters in the Talysh fold zone of the Lesser Caucasus (Azerbaijan) based on isotope-geochemical data (3Не/4Не, $${\delta ^{13}}{C_{c{o_2}}},{\delta ^{13}}{C_{c{h_4}}},{\delta ^{15}}{N_{{N_2}}}{,^{87}}Sr{/^{86}}Sr,\delta {D_{{H_2}O}},and{\delta ^{18}}{O_{{H_2}O}}$$ δ 13 C c o 2 , δ 13 C c h 4 , δ 15 N N 2 , 87 S r / 86 S r , δ D H 2

Abstract

It is shown that the gas and water phases of the thermal nitrogen–methane waters in the Talysh fold zone of the Lesser Caucasus mountain system contain helium and strontium with mantle isotope signatures (3Не/4Не from 200 × 10–8 to 401 × 10–8 and 87Sr/86Sr from 0.70490 to 0.70562). At the same time, clear signs of the mantle component in other gases (nitrogen, methane, and carbon dioxide) are absent. The δ15N value in nitrogen varies from +0.3 to +1.7‰, methane is mainly characterized by δ13C from–57.4 to–38.0‰, while δ13C(CО2) varies from–24.4 to–11.3‰. An increase of the CО2 content is accompanied by the decrease of δ13C in CО2, against the background of increasing SO4 content in the salt composition of waters. This indicates a microbial nature of CO2 in the studied gases. Thus, the presence of mantle helium and strontium in the thermal waters is likely related to their leaching from the Pleogene–Neogene host volcanic rocks. The studies of the oxygen and hydrogen isotope composition in water revealed quite different mechanisms for the formation of cold and thermal waters of the region. The cold waters are mainly fed by local infiltration, whereas the feeding of thermal nitrogen–methane waters is strongly provided by transit atmogenic waters (>50%), which are formed in the mountain ranges at altitudes no less than 1600 m and spaced at 20–40 km or more from the thermal discharge sites.